Step 1: Line an 8x8 inch square baking pan with parchment paper or foil, leaving a generous overhang on the sides. Lightly grease the paper/foil.
Step 2: In a medium heatproof bowl, combine 1 cup of the sweetened condensed milk and 2 cups of semi-sweet chocolate chips.
Step 3: Microwave on high for 1 minute, then stir well with a spatula. Continue microwaving in 30-second intervals, stirring thoroughly after each, until the mixture is completely smooth and melted (about 1.5-2 minutes total).
Step 4: Once smooth, stir in 1/2 teaspoon of vanilla extract and a pinch of salt until well combined. Set aside.
Step 5: In a separate medium heatproof bowl, combine the remaining 1 cup of sweetened condensed milk and 1 cup of creamy peanut butter.
Step 6: Microwave on high for 1 minute, then stir well. Continue microwaving in 30-second intervals, stirring thoroughly after each, until the mixture is completely smooth and melted (about 1.5-2 minutes total).
Step 7: Once smooth, stir in the remaining 1/2 teaspoon of vanilla extract until well combined.
Step 8: Pour about two-thirds of the chocolate fudge mixture into the prepared pan and spread evenly across the bottom.
Step 9: Dollop spoonfuls of the peanut butter fudge mixture randomly over the chocolate layer.
Step 10: Carefully spoon the remaining chocolate fudge mixture into the gaps and on top of the peanut butter fudge dollops.
Step 11: Using a butter knife or a skewer, gently swirl the chocolate and peanut butter mixtures together to create a marbled effect. Do not overmix; aim for distinct swirls.
Step 12: If desired, sprinkle with optional toppings like flaky sea salt, chopped peanuts, or festive sprinkles for extra flair.
Step 13: Cover the pan tightly with plastic wrap and refrigerate for at least 2 hours, or until the fudge is completely firm. This is crucial for clean cuts.
Step 14: Once firm, use the parchment paper overhang to lift the fudge out of the pan. Place it on a cutting board and, using a sharp, warm knife, cut into 1-inch squares. Store in an airtight container at room temperature or in the refrigerator for up to 2-3 weeks.